This invention relates to drive assemblies, and, more particularly, to a drive assembly for rotating the centerpost of a power tire changing machine.
The majority of tires used on passenger cars and light trucks are of the tubeless type which are mounted on wheels having opposed outer rim flanges located adjacent upper and lower bead seats which lead to an inner annular area, usually of the drop center type. The sidewalls of the tire each terminate in an annular section or bead which are adapted to seat within the upper and lower bead seats of the wheel. The tire beads sealingly engage the bead seats of the wheel to prevent leakage of air and dislocation of the tire from the wheel.
Tire mounting and demounting machines have been in use for several decades for the removal of tubeless tires from a wheel. In demounting a tire from a wheel, the tire and wheel are first placed on the tire changing machine over a centerpost and then clamped to the support table of the machine in an essentially horizontal position. The outer face or sidewall of the tire faces upwardly so that its valve core may be removed to enable the air pressure within the tire to drop to about atmospheric level.
The next step in removing the tire from the wheel is to force both the upper and lower tire beads off of their bead seats on the wheel toward the inner drop center of the wheel. Most tire changing machines in commercial use today employ upper and lower bead breaking shoes which engage the beads on opposite sides of the tire and are forced inwardly to push the tire beads off of their bead seats. At this point in the demounting operation, the beads on both sides of the tire are displaced from the bead seats of the wheel and are positioned near the center of the wheel.
The final step in the demounting process is to force at least one bead and sidewall of the tire over the outer flange of the wheel. This is accomplished with a wedge or prying tool which is operatively mounted atop the rotatable centerpost shaft of the tire changing machine. The prying tool is formed with a slotted end adapted to fit over a flattened portion at the top of the centerpost shaft. The other end of the tool extends downwardly from the top of the centerpost shaft over the outer flange of the wheel and beneath the upwardly facing bead of the tire. The prying tool is rotatable with the centerpost shaft along the outer rim of the wheel and forces the tire bead over the outer rim as it rotates.
In prior tire changing machines, the centerpost shaft is usually rotated by a single fluid motor such as a pneumatic or hydraulic cylinder having a reciprocating piston. When the cylinder is actuated by air or liquid, the piston is extended and retracted to rotate the centerpost shaft, and, in turn, the prying tool. In some machines, actuation of the cylinder through a complete cycle, in which the piston is moved from a fully retracted to a fully extended position, results in rotation of the centerpost shaft through an angle ranging from about 270.degree. to 315.degree.. In such machines, the prying tool is removed after one cycle so that the cylinder may be retracted in preparation for another demounting operation. Other machines do not recycle but utilize cylinders which develop power upon both extension and retraction of the piston. These machines, however, are also limited to rotation of the centerpost shaft between 270.degree. and 315.degree. both during full extension and retraction of the piston.
Several problems have resulted from the use of centerpost drive assemblies of the type described above. In many tire demounting operations, particularly with tires of larger diameter, a substantial torque is required to rotate the centerpost so that the prying tool can urge the tire bead and sidewall above the outer flange of the wheel. A standard size pneumatic or hydraulic cylinder may be incapable of providing sufficient torque for some applications. While the problem of insufficient torque can be avoided by employing a very large cylinder, this solution adds substantially to the expense and size of the tire changing machine.
Another disadvantage of the centerpost shaft drives of prior art tire changing machines is that they do not rotate the centerpost shaft beyond 270.degree. to 315.degree., and also are not reversible to permit rotation of the centerpost both in a clockwise and counterclockwise direction. As mentioned above, in most prior art centerpost drives a complete cycle of operation of the cylinder results in rotation of the centerpost through a maximum angle of only about 270.degree. to 315.degree.. If the tire bead has not been pryed above the outer flange of the wheel by the time the centerpost shaft has completed its rotation, the pry bar must be removed and the cylinder recycled. Often, in order to remove the pry bar, which is wedged between the tire bead and wheel, a hammer or other tool must be used to forcefully release the tool from beneath the tire bead. Since prior art centerpost shaft drives permit rotation of the centerpost in only one direction, jamming of the pry bar cannot be overcome by reversing the direction of rotation of the centerpost.